Hitherto, electroluminescent display, hereinafter referred to as ELD, is known as a light emission type electronic displaying device. As the constitution element of the ELD, inorganic electroluminescent devices and organic electroluminescent devices, hereinafter also referred to as organic EL device, can be cited. The inorganic electroluminescent device which is applied as flat type light sources requires a high alternating-current voltage for driving as the light emitting device.
Meanwhile, the organic EL device is composed of a cathode, an anode and a light emission layer containing a light emission substance placed between the cathode and the anode, and is a light emission device in which electrons and positive holes are injected into the light emission layer to form excitons by recombination thereof and light emitted by inactivation of the excitons (fluorescence or phosphorescence) is utilized. Therefore, light can be emitted by applying a voltage of from several volts to several tens volts. The device displays wide view angle and high visibility since which is a self light-emitting device and is noted from the viewpoint of space saving and portability because the device is a thin layer type complete solid state element.
Development in the future of application of the organic EL device, an organic EL device capable of emitting high luminance light with higher light emitting efficiency at lower electric power consumption is demanded. As such development, a method for raising luminance of emitted light and prolonging lifetime of the device by doping a stilbene derivative, a distyrylarylene derivative or a trisstyrylarylene derivative with slight amount of a fluorescent substance, cf. Patent Publication 1 for example, a device having an organic light emission layer doped with slight amount of a fluorescent substance, cf. Patent Publication 2 for example, and a device having an organic light emission layer composed of a 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye, cf. Patent Publication 3 for example, are known.
In the technologies described in the above documents, it is said that the limit of external quantum efficiency (ηcxt) is 5% because the formation probability of the light emission exciton is 25% since the formation ratio of singlet exciton to triplet exciton is 1:3. And when the light from the singlet state exciton is used, its external output efficiency of the light is about 20%.
However, investigation on materials capable of emitting phosphorescence at room temperature is accelerated; cf. Non-patent Publication 2 and Patent Publication 4 for example, since an organic El device utilizing phosphorescence emitted from the triplet state of the exciton is reported by Princeton University, cf. Non-patent Publication 1 for example. The upper limit of the internal quantum efficiency can be raised by 100% when the triplet sate is utilized. Therefore, the light emission efficiency becomes up theoretically by four times compared with the emission by utilizing of the singlet state so that the performance becomes approximately the same as that of a cold cathode ray tube. Accordingly, such device is received attention because it can be applied as an illumination light source. For example, many compounds, principally heavy metal compounds such as iridium type complexes, are synthesized and investigated, cf. Non-patent Publication 3 for example.
At the present time, improvement in the light emission efficiency and prolongation in the lifetime of the organic El device utilizing the phosphorescent light emission are actively investigated. As one of those, a device having a plural layer constitution can be cited. On early days, the organic EL device has a single layer constitution in which the light emission layer is placed between the anode and the cathode. In such case, all processes of carrier injection, transfer of the carrier and light emission should be carried out only in the light emission layer. Therefore, the light emission efficiency is very low. Thereafter, a device having plural layers (multi-layered structure) which separately have functions such as the carrier injection, carrier transfer, blocking and light emission is developed so that large progress in the raising efficiency, prolongation of life and lowering in the driving voltage is obtained.
On the other hand, the vapor deposition process which is usually applied in the production of the organic EL device using a low molecular weight compound causes problems of production equipment and energy efficiency when the organic EL element having enlarged area is manufactured, and it is thought that a printing method including a ink-jet printing method and a screen printing method or a coating method such as a spin coating method and a cast coating method are desirable.
For example, plural light emission substances different in the maximum wavelength of emission light from each other should be contained in the light emission layer when the device is a white light emission device. Particularly, in the case of the phosphorescent light emission device, it is difficult to deposit plural phosphorescent dopants in the same ratio in every times of manufacturing so as to be surmised to cause a problem of production yield in the manufacturing process. However, if manufacturing of the organic EL device by the printing method or the coating method can be carried out by using a material superior in the solubility in a solvent, it is possible to contain phosphorescent dopants in the same ratio into every Organic EL devices to be manufactured by preparing a solution containing the phosphorescent dopants in the same ratio and the white color light emission devices each emitting the same color light can be stably manufactured.
As above-mentioned, the coating manufacturing method is generally inferior to the vapor deposition method though the coating method is considered as superior one of manufacturing methods. Therefore, the coating method is demanded to be further improved. For example, there is a problem on the coating method that adhesion between the layers is lowered, and a method is described, cf. Patent Publication 5 for example, in which the plural layers are subjected to pressing, heating and drying at once after coating for improving the adhesion so as to form irregularities at the interface between the organic layers.
Moreover, it is very difficult to form uniformly piled organic layers by the coating method when the plural organic layers each different in the function thereof such as a positive hole transfer layer, a light emission layer and an electron transfer layer are piled for raising the light emission efficiency and prolonging the lifetime, because the surface of the lower layer tends to be dissolved by the solvent of the upper layer coating liquid so that the lower layer is confused.
Contrary to that, a method is disclosed, cf. Patent Publication 6 for example, in which materials of the upper layer is dissolved in a solvent having a solubility parameter without the range of that of the principal material constituting the lower layer so as to pile the layers without confusion at the surface of the lower layer, cf. Patent Publication 6 for example. Moreover a method is disclosed, cf. Patent Publication 7 for example, in which the materials of the upper layer is dissolved in a mixture a solvent (poor solvent) being without the dissolvable range of the solubility parameter of the principal material constituting the lower layer and a solvent (good solvent) being within the dissolvable range of the solubility parameter of the material of the upper layer for lowering the solubility, cf. Patent Publication 7 for example.    Patent Publication 1: Japanese Patent No. 3093796    Patent Publication 2: JP-A 63-264692    Patent Publication 3: JP-A 3-255190    Patent Publication 4: U.S. Pat. No. 6,097,147    Patent Publication 5: JP-A 2005-26000    Patent Publication 6: JP-A 2002-299061    Patent Publication 7: JP-A 2005-259523    Non-patent Publication 1: M. A. Baldo et al., Nature, 395, p.p. 151-154 (1998)    Non-patent Publication 2: M. A. Baldo et al., Nature, 403, 17, p.p. 750-753 (2000)    Non-patent Publication 3: S. Lamansky et al. J. Am. Chem. Sci., 123, p. 4304 (2001)